1. Field of the Invention
This invention relates to a method of developing images in which a developer containing carriers and toners is supplied from a developer reservoir to the surface of a developer conveyor so that a developer layer can be formed, and the toners are flown from the developer layer formed on the conveyor by utilizing the application of a bias voltage so that an electrostatic latent image formed on the surface of an image forming body can be developed; and this invention also relates to an image recording apparatus utilizing the abovementioned method.
2. Description of the Prior Art
Magnetic brush developing methods are already known, in which an electrostatic latent image is developed by making use of such a developer as is so-called two-component type developer containing magnetic carrier particles and toner particles, also if necessary, a small amount of hydrophobic silica and the like, and by bringing a developer layer formed on a developer conveyor (hereinafter called as a sleeve, for convenience.) into contact with the surface of an image forming body (hereinafter called as a photoreceptor, for convenience.); and electrostatic recording apparatuses including electrophotographic copying apparatuses are also already known, in which the abovementioned methods are utilized respectively. In these methods and apparatuses, however, there are instances where an image density is lowered in a low toner-concentration and a non-image produced area is soiled with fog in a high toner-concentration, even if the requirements for forming an image such as a D.C. bias voltage to be applied to a sleeve are adjusted at the time when the toner concentration of a toner developer was varied in the developer reservoir of a developing unit. There is also a limit to keep a proper toner concentration, that is, 3 to 10% by weight thereof to the total amount of developer. Hereinafter, the proportion of toner to developer will be called as a toner concentration, because it will be expressed as a percentage by weight.
In the meantime, when utilizing a developing method under non-contact conditions in which toner is flown from a developer layer laid on a sleeve in a vibrating-electric field to the surface of a photoreceptor so as to be adhered thereto without bringing the developer layer into contact with the surface of the photoreceptor, then it can clearly be proved by a lot of experiments that the density of image developed is varied according to the concentration of toner in developer as seen in FIG. 1.
In FIG. 1, it is found that the density of image developed, that is, the density of image recorded, tends to be increased with the increase of the concentration of toner when the concentration of toner in developer is in a relatively lower range, and reversely the density of image developed tends to be decreased when the concentration of toner is excessively increased. It may be estimated that the phenomenon that the density of an image developed is decreased may be caused by not satisfactorily charging toners stored in a developer reservoir when the concentration of toner is excessively increased. In other words, it may be estimated that the phenomenon may be caused by the decrease of the proportion of the toner which is satisfactorily charged. The abovementioned developing method to be carried out under non-contact conditions, on the other hand, can enjoy some great merits, for example, that is can hold down a fog to a very low degree, and that the allowable range of the concentration of toner in developer may be widened to the order of 5 to 50% by weight (See the left hand side of the curve from the peak thereof in FIG. 1). These merits indicate that this method may be used without replenishing toners provided that there is a suitable image density adjusting means, even if the concentration of toner adjusted in advance to the order of 30 to 50% by weight should be decreased to the order of 10% by weight.
According to the second method relating to this invention, stable images without having any fog may be obtained even if a developer having the concentration of toner of 10 to 90% by weight. In contrast therewith, in the traditional magnetic brush developing method described above, a variation of toner concentration will immediately cause the variation of image density, because the allowable range of the concentration of toner in developer is limited.
For the purpose of preventing the variations of density of image developed, it must be necessary to provide the conventional developing methods each (1) with a toner supply system in which the electrostatic latent image of a standard density image is formed separately from a recorded image on a photoreceptor and the standard density of the electrostatic latent image developed is constantly measured by a photocoupler or the like so as to judge the concentration of toner in developer, for example, the concentration is judged to be insufficient when the standard density of image developed becomes lower than a prescribed density, and fresh toners are constantly replenished to a developer reservoir so that the standard density of image developed may constantly be kept higher than the prescribed density.
Or, it must be necessary to provide the conventional methods each (2) with a control means in which the inductance of developer is constantly measured by an inductance measuring means arranged to the inside of a developer container and toners are replenished to a developer reservoir so that the inductance may be kept at a prescribed level. Concentration of toner in developer is kept constant in a limited range by the control means and the other image forming requirements are also kept constant so that sharp recorded image without having any fog can stably be obtained.
However, in such a method that toner replenishments are controlled so as to keep the concentration of toners in developer constant and the density of image developed, i.e., the density of image recorded, may thus be maintained, the toner replenishing device will be complicated and expensive. It is also required the accuracy when measuring the standard density of image developed in the abovementioned item (1), and when measuring the inductance of a developer in the item (2). In addition, in such a method as the item (1) in which the concentration of toner is obtained indirectly, there is also such a problem as that superfluous toner replenishments may sometimes be carried out by misunderstanding a lowering of the density of toner developed caused by other factors as a lowering of the concentration of toners.
Accordingly, there have so far been demanded strongly, from the viewpoint of image quality, for a developing method capable of obtaining long-life, stable and sharp-cut images recorded by making use of a two-component type developer without necessarily keeping it under an extremely strict control; and for the recording apparatus thereof.